System and method for locating nodes within a wireless network

ABSTRACT

A system and method for determining the location of nodes within a network. Beamforming coefficients associated with signals transmitted from a first node to a second node are determined and, based on the beamforming coefficients, direction is determined between the first and second nodes. Distance is determined as a function of direction and the location of the second node is determined as a function of distance and direction.

BACKGROUND

Wireless local area networks (LANs) such as Wi-Fi provide high-speeddata distribution for both home and business. With the advent of newmechanisms for improved signal quality enabling higher speeds, Wi-Fi hasbecome one of the most universally accepted wireless distributionmechanisms for the wireless LAN service space. As we have become morereliant on Wi-Fi for higher speeds and extended coverage, it has becomemore important to be able to monitor the Wi-Fi network for performancedegradation and to apply corrections.

One of the methods for improving signal quality in Wi-Fi LANs isbeamforming. Beamforming can be likened to phased array signalgeneration, commonly used in radar, in that by adjusting the relativephases of an array of signals emanating from an array of antennas, thecombined signal can be formed such that the phases of each antennasignal will constructively interfere at the desired target.

Beamforming has become a standard option with the advent of IEEE 802.11ac, which includes specific protocol to enhance the ability of an AccessPoint (AP) and Station (STA) to effectively use what is called “explicitbeamforming.” Explicit beamforming requires that the STA and APcommunicate received signal information over that protocol to optimizethe beamforming process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example of a wireless network that can implementvarious techniques of this disclosure.

FIG. 2 illustrates a method of determining a location of a target clientin the wireless network of FIG. 1.

FIG. 3 illustrates another example of a wireless network that canimplement various techniques of this disclosure.

FIG. 4 illustrates another method of determining direction and locationof a target client in a wireless network.

SUMMARY

In general, this disclosure is directed to techniques for determining alocation of devices, e.g., Stations (STAs), participating in a Wi-Finetwork.

In an example, this disclosure is directed to, in a network having afirst and second node, wherein each node includes beamformingcoefficients, a method of determining the location of a node, the methodcomprising determining the beamforming coefficients associated withsignals transmitted from the first node to the second node, determininga direction of the second node using the beamforming coefficients,determining a distance between the first node and the second node as afunction of time of flight and direction, and determining a location ofthe second node as a function of distance and direction.

In another example, this disclosure is directed to, in a network havingincluding a first, second and third node, wherein each node includesbeamforming coefficients, a method of determining the location of anode, the method comprising determining the beamforming coefficientsassociated with signals transmitted from the first node to the secondnode and the beamforming coefficients associated with signalstransmitted from the third node to the second node, and determining adirection of the second node from the first and third nodes using thebeamforming coefficients, and determining a location of the second nodeas a function of direction from each of the first and third nodes andthe location of the first and third nodes.

DETAILED DESCRIPTION

This disclosure is directed to techniques for determining a location ofdevices, e.g., Stations (STAs), participating in a Wi-Fi network.Existing techniques utilize three or more access points with knownlocations to triangulate STAs within an indoor environment. Thesetechniques can be sufficient for indoor environments such as shoppingmalls, airports or businesses, for example, with many access pointsavailable for triangulation. However, these techniques do not work inspaces where only one or two access points are available. With just oneor two access points, it is impossible to use simple triangulationbecause three or more access points are needed for triangulation.

In addition, when STAs participate in a Wi-Fi network within an indoorenvironment (for example, homes, stores, and coffee shops), locationmechanisms such as Global Positioning Satellite (GPS) location detectionmay not be available to locate the STAs.

The techniques described in this disclosure solve the problem where onlyone or two access points are available. In addition, the techniques inthis disclosure can enhance triangulation accuracy when three or moreaccess points are available. This disclosure describes using locatingtechniques that utilize beamforming information and round trip delay(RTD) information to obtain position information without the use of GPS.

As mentioned above, beamforming has become a standard option for Wi-Finetworks with the advent of IEEE 802.11 ac. The IEEE 802.11 ac standardincludes the ability of an Access Point (AP) and Station (STA) to use“explicit beamforming” to improve signal-to-noise in communicationsbetween APs and STAs. With explicit beamforming, the STAs and APscommunicate received signal information in order to optimize thebeamforming process.

Beamforming directs a radiated signal power in the direction of thetarget client, e.g., STA, and can improve the signal-to-noise ratio atthe target client's receive antennas. Beamforming can also be used in areceive sense to “null” the interfering signals that would impair thesignal-to-noise ratio at the receiver. With transmit or receivebeamforming, there is a radiated (or received) signal pattern in threedimensions such that the strongest signal lobe is desirably directed tothe target client. The direction of the strongest signal lobe can bedetermined with relation to the Wi-Fi AP or STA within a threedimensional axis coordinate system aligned with the AP or STA using thebeamforming coefficients and a Wi-Fi radio/antenna system that has beencharacterized (e.g., calibrated).

FIG. 1 depicts a wireless network 100 that can implement varioustechniques of this disclosure. The wireless network 100, e.g., a Wi-Finetwork, can include a single AP 102 (first node) and a single STA 104(second node). An access point, such as AP 102, is a wirelesscommunication device that, in some examples, can be considered to be,and can include at least some of the functionality of, a base station.An access point can provide access for one or more stations, e.g., STA104, to a wired network. A station, such as STA 104, can be a fixed ormobile wireless communication device, such a laptop or portable computerwith wireless communication capability, a smart phone, a tabletcomputer, a personal digital assistant, a wireless telephone, a wirelessheadset, a television, a set-top box, or other device that may receiveand/or transmit information wirelessly.

AP 102 and STA 104 can communicate using Multiple-Input/Multiple-Output(MIMO) transmission protocols in accordance with an IEEE 802.11standard, such as the IEEE 802.11ac standard. As seen in FIG. 1, the AP102 can include a plurality of antennas, e.g., four antennas, showngenerally at 106. The antennas 106 can be, for example, omnidirectionalantennas. The AP 102 can transmit a MIMO transmission 108 to the STA104. The STA 104 can include a plurality of antennas, e.g., fourantennas, shown generally at 110. The antennas 110 can be, for example,omnidirectional antennas. The STA 104 can transmit a signal back to theAP 102 on a plurality of communication paths, e.g., four communicationpaths.

The AP 102 can include a processor 112, a transmitter 114, a receiver116, a direction calculation circuit 118, and a distance calculationcircuit 120. The transmitter 114 can include a beamformer circuit 122.For each of the plurality of antennas 106, the processor 112 candetermine beamforming coefficients of a signal to be transmitted, e.g.,a phase and amplitude, by the transmitter 114, e.g., based on receivedsignal strength. The beamformer circuit 122 of the transmitter 114 canapply the beamforming coefficients to the signal to be transmitted. Byapplying the beamforming coefficients to the signal to be transmitted,the AP 102 can steer the strongest signal lobe toward the STA 104.Similarly, the receiver 116 can include a beamformer circuit 124. Foreach of the plurality of antennas 106, the beamformer circuit 124 of thereceiver 116 can apply the beamforming coefficients to the receivedsignal.

The STA 104 can include a processor 130, a transmitter 132, a receiver134, a direction calculation circuit 136, and a distance calculationcircuit 138. The transmitter 132 can include a beamformer circuit 140,and the receiver 134 can include a beamformer circuit 142. Thesecomponents of STA 104 are similar to those of AP 102 and, for purposesof conciseness, will not be described again.

The techniques described in this disclosure use beamforming informationto determine a direction of a received signal and round trip delay (RTD)information to determine a distance of a received signal, therebyobtaining position information of the STA 104, without the use of GPS,where only two devices, e.g., one AP and one STA, are available. Usingbeamforming techniques, the direction calculation circuit 118 of the AP102 can determine the direction of arrival (DOA) of a signal source,e.g., a signal transmitted by STA 104, based on the strength of phasesof signals received by the antennas 106.

Once the AP 102 has the direction to a device such as a STA 104, it candetermine a distance to the device. To estimate a distance to the STA104, the distance calculation circuit 120 of the AP 102 can use RTDinformation. For example, the distance calculation circuit 120 candetermine RTD information based on the difference in time between atransmitted Request-to-Send (RTS) signal and a received Clear-to-Send(CTS) Wi-Fi signal, e.g., using IEEE standard 802.11. Using thecalculated RTD information, the distance calculation circuit 120 canestimate a distance since the electromagnetic signals between the AP 102and the STA travel at a speed very close to the speed of light.

In other embodiments, distance can be calculated via a four-waytime-of-arrival (TOA) determination such as is described by Hoene in LowCost WLAN based Time-of-flight Trilateration, Precision Indoor PersonnelLocation and Tracking for Emergency Responders, Third Annual TechnologyWorkshop, Aug. 5, 2008. Hoene details two different four-way time offlight (TOF) calculations based on the use of a combination of RTS, CTS,a data transmission and an ACK to determine a round-trip time of flightthat can be used to determine distance between objects. Hoene alsodetails ways to overcome quantization due to clock frequencies intypical APs 102 and STAs 104. The description of four-way TOFcalculations, the use of a third node to monitor transmissions andcalculate time of flight, and the techniques for overcoming the effectsof quantization are incorporated herein by reference.

In one embodiment, the AP 102 can transmit an RTS, receive a CTSresponse back from the STA 104, transmit a data packet to the STA 104and receives back an ACK from the STA 104. In such an embodiment,distance is a function of time of flight (t_(TOF)) for two trips from AP102 to STA 104 and two trips back and can be calculated as:

t _(TOF)=¼*(t _(3S) −t _(0S) −t _(DATA) −t _(CTS) −t _(RTS)−3t _(SIFS))

where t_(3S) is the time the ACK is received at AP 102, t_(0S) is thetime RTS is sent by the AP 102, t_(DATA) is the time needed to transmitthe data packet, t_(CTS) is the time needed to transmit the CTS signal,t_(SIFS) is the short interframe space (SIFS) period, and t_(RTS) is thetime needed to transmit the RTS signal. In one embodiment, successivemeasurements of t_(TOF) are binned and used to generate a continuousdistribution whose center is indicative of t_(TOF) without thequantization effects.

FIG. 2 illustrates a method of determining a location of a target clientin the wireless network of FIG. 1. In the example approach of FIG. 2,the AP 102 determines the beamforming coefficients associated withsignals to be transmitted to the STA 104 (block 200). The AP 102calculates a direction to the STA 104 using the beamforming coefficientsfor transmissions to STA 104 (block 202). The AP 102 can then determinea distance to the STA 104, e.g., using RTD information (block 204). Inthis manner, the AP 102 can determine a location to the STA 104.

In some embodiments, the AP 102 can vary its beamforming coefficientsover time as it tries to maintain the highest possible signal strength.In some such embodiments, the AP 102 can determine a change in directionwhen the beamforming coefficients change more than a threshold amount.The AP 102 can determine the beamforming coefficients associated withsignals to be transmitted to the STA 104 (block 206). Then, the AP 102,e.g., the processor 112, can compare the beamforming coefficientsassociated with signals to be transmitted to the STA 104 to a threshold,for example, to determine whether the coefficients have changed enoughto require a new determination of distance and direction (block 208).

If the coefficients are outside the threshold (“YES” branch of block208), control can move to block 202, and the AP 102 can calculate adirection to the STA 104 as a function of the beamforming coefficientsfor transmissions to STA 104.

If, however, the beamforming coefficients associated with signals to betransmitted to STA 104 have not changed enough to require a newdetermination of direction or distance (“NO” branch of block 208),control can move to block 206, and AP 102 once again can determine thebeamforming coefficients associated with signals to be transmitted tothe STA 104.

FIG. 3 depicts another wireless network 300 that can implement varioustechniques of this disclosure. FIG. 3 depicts one access point, namelyAP 102 (first node), and two target clients, namely a STA 104A (secondnode) and a STA 104B (third node). The STAs 104A, 104B are similar tothe STA 104 of FIG. 1 and will not be described in detail. As seen inFIG. 3, the AP 102 can communicate with the APs 104A, 104B using MIMOtransmissions 108A, 108B, respectively. In addition, the STAs 104A, 104Bcan communicate with one another using Wi-Fi signals 150.

In accordance with this disclosure, beamforming techniques can becombined with triangulation techniques to more accurately determinelocation information for two or more target clients, e.g., STA 104A andSTA 104B. Using the techniques described above, the AP 102 can determineestimated distance and direction information to both the STA 104A andthe STA 104B.

In addition, in some example implementations, the STA 104A can determinea distance to the STA 104B, e.g., using Wi-Fi signals 150 via the RTDtechnique. In some embodiments, TDLS (Tunneled Direct Link Setup) can beused to establish STA to STA direct communication and again, determineRTD. In one embodiment, STAs 104A, 104B can use RTD to determine STA toSTA separation. In another embodiment, STAs 104A, 104B can usebeamforming coefficients to determine signal direction to other STA 104(not depicted). In yet another embodiment, STAs 104A, 104B can use RTDto determine STA to STA separation and use beamforming coefficients todetermine signal direction.

One or both of the STA 104A, 104B can transmit the determined distanceinformation between STA 104A and STA 104B to the AP 102. Using theadditional information, the distance calculation circuit 120 of the AP102 can recalculate the distances to the STA 104A and the STA 104B toincrease the resolution of the calculation.

In some embodiments, the AP 102 can monitor transmissions between theSTA 104 and the STA 104B, and the AP 102 can calculate a time of flight(t_(TOF)) in order to determine a distance. In one such embodiment, theSTA 104A can transmit an RTS to the STA 104B, receive a CTS responseback from the STA 104B, transmit a data packet to the STA 104B, andreceive back an ACK from the STA 104B. In such an embodiment, the AP 102can monitor the RTS, CTS, data, and ACK transmission and can calculate adistance that is a function of time of flight (t_(TOF)) for two tripsfrom STA 104A to STA 104B and two trips back. For example, the AP 102can a time of flight by:

t _(TOF)=½*(t _(2M) −t _(0M) −t _(CTS) −t _(RTS)−2t _(SIFS))

where t_(2M) is the time the data packet is received at the monitoringAP 102, t_(0M) is the time RTS is received by the monitoring AP 102,t_(CTS) is the time needed to transmit the CTS signal, t_(RTS) is thetime needed to transmit the RTS signal, and t_(SIFS) is the shortinterframe space (SIFS) period. As in above, in one embodiment,successive measurements of t_(TOF) are binned and used to generate acontinuous distribution whose center is indicative of t_(TOF) withoutthe quantization effects. A distance can be calculated based on the timeof flight.

The location information has many uses. In some embodiments, it can beused to diagnose Wi-Fi problems due to STA placement. In someembodiments, it can be used to locate illegitimate STA connections onnetwork 100. In some embodiments, it can be used to determine whetherthere is interference from devices such as APs and STAs that are notparticipating in a network, e.g., networks 100, 300, and to determinewhich of the devices are creating the most significant interference.

Finally, it can be used in some embodiments to identify the location ofspecific users. In some such embodiments, the information about a user'slocation can be used for targeted advertising and offers.

It should be apparent that, in open space, the above methods can do agood job determining the direction of each node from other nodes and thedistance between nodes. Reflections may reduce the accuracy of both thebeamforming location and the RTD calculations. If one or more nodes arein a known location, the location of the nodes in three-dimensionalspace can also be determined. Therefore, by using beamforminginformation, distance measurements and triangulation from multipleelements with known locations in the network, one can obtain positioninformation without the use of GPS.

In some embodiments, the accuracy of the time of flight (t_(TOF))calculation can be enhanced if certain stations STA 104A, 104B on theWi-Fi network have known locations. Such information can be used to helptriangulate the locations of other STAs 104 in conjunction with theinformation from access point 102. In some embodiments, it can also beused to determine if the distance and direction determined for aparticular path is a direct line of sight, or a product of one or morereflections.

In one embodiment, the AP 102 can maintain a table of locations, e.g.,in a memory, that have been determined for one or more stations, e.g.,STAs 104A, 104B. Each time the AP 102 determines a new location for astation, that location can be compared to the location previouslydetermined by the AP 102 or by other stations to see if the location isconsistent with what was previously determined. If not, in someembodiments, the AP 102 can determine the most likely location based onthe distance and direction information as measured by the AP 102.

In other embodiments, each node (whether an AP 102 or a STA 104A, 104Bcan) can maintain a table of locations that have been determined for allstations and for the AP 102. Each time a node, e.g., a station or an AP,determines a new location for another node, that location can becompared to the location determined by one or more other nodes to see ifthe location is consistent with what was determined by other nodes, oreven with a location measured previously by the same node. If not, insome embodiments, the node can determine the most likely location basedon the distance and direction information as measured by each of thenodes.

FIG. 4 illustrates another method of determining direction and locationof a target client in a wireless network. In the example approach ofFIG. 4, the AP 102 can determine the beamforming coefficients associatedwith signals to be transmitted to, for example, the STA 104 (block 400).The AP 102 can calculate a direction to the STA 104 using the determinedbeamforming coefficients for transmissions to the STA 104 (block 402).

The AP 102 can determine a distance to the STA 104 and then the locationof the STA 104 using the determined distance and direction and thelocation of AP 102 (block 404). The AP 102, for example, can determinewhether the newly determined location information is consistent withprevious location determinations (block 406). If a determination is madethat the location measured is consistent with previous locationdeterminations, (“YES” branch of block 406), control can move to block410 and the location information can be saved. In some embodiments, thelocation information can be shared with other nodes.

If a determination is made that the location measured is not consistentwith previous location determinations, (“NO” branch of block 406), thenthe most likely location is determined as a function of the previouslycalculated location, distance and direction information (block 408).Control then moves to block 410 and the location information can besaved. In some embodiments, the location information can also be sharedwith other nodes.

The AP 102 can again determine the beamforming coefficients associatedwith signals to be transmitted to the STA 104 for example (block 412).Next, the AP 102 can determine if the beamforming coefficientsassociated with signals to be transmitted to STA 104 have changed enoughto require a new determination of distance and direction, e.g., whetherthe change is outside a threshold, (block 414). If so (“YES” branch ofblock 414), then control moves to block 104B, and AP 102 can calculate adirection to the STA 104 using the beamforming coefficients fortransmissions to the STA 104.

If, however, the beamforming coefficients associated with signals to betransmitted to the STA 104 have not changed enough to require a newdetermination of direction or distance (“NO” branch of block 414),control moves to block 412, and the AP 102 once again can determine thebeamforming coefficients associated with signals to be transmitted toSTA 104.

A similar approach can be used to detect the location of any of theother stations and to update their tables of node locations.

In some embodiments, AP 102 and STA 104 can include sensors such asaccelerometers for detecting when they are moved, the direction they aremoved and the distance they are moved. In some such embodiments, a newlocation can be calculated based on the measured movement and a locationtable can be updated accordingly. In some embodiments, the new locationmeasured can be updated as necessary based on subsequent distance anddirection measurements and location determinations by other nodes.

In some embodiments, location is determined periodically. In some suchembodiments, a moving node can be checked for present location morefrequently than a static node.

In some environments, multipath is more of a concern. High levelreflective signatures make pure direction and time of flight parametersalone inadequate to accurate triangulation of client positions. In someembodiments, beams from signals in two different spectrums can be usedto determine distance and direction. In some such embodiments, beamsbetween the two spectrums can be correlated, since 2.4 GHz is more aptto penetrate and 5 GHz is more apt to reflect, so one can better assesswhether the direction and time of flight is direct or indirect whencommunicating between dual-band end-points AP 102 or STA 104. Inaddition, the two different bands often use different sets of antennas,so it is possible to use the two bands to reduce systematic errors dueto sampling at a given frequency. Systematic errors of each band aretypically not correlated.

As noted above, GPS has limited capability to detect the location ofdevices within indoor environments while triangulation alone requiresdirection measurements from three or more nodes with known locations. Byusing beamforming information, RTD and triangulation from multipleelements with known locations in the network, position information maybe obtained without the use of GPS and in a way that is more accuratethan triangulation alone. Furthermore, the method works even if only oneor two nodes are available to determine location of the nodes having anunknown location.

What has been discussed above is the use of beamforming coefficients andRTD information to determine the location of a node within a wirelessnetwork. Such an approach takes advantage of the use of beamformingprotocols inherent to many node radios. In some embodiments, accesspoints such as residential gateways determine the physical location ofdevices trying to access the access points.

The above detailed description includes references to the accompanyingdrawings, which form a part of the detailed description. The drawingsshow, by way of illustration, specific embodiments in which theinvention can be practiced. These embodiments are also referred toherein as “examples.” Such examples can include elements in addition tothose shown or described. However, the present inventors alsocontemplate examples in which only those elements shown or described areprovided. Moreover, the present inventors also contemplate examplesusing any combination or permutation of those elements shown ordescribed (or one or more aspects thereof), either with respect to aparticular example (or one or more aspects thereof), or with respect toother examples (or one or more aspects thereof) shown or describedherein.

In the event of inconsistent usages between this document and anydocuments so incorporated by reference, the usage in this documentcontrols.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one, independent of any otherinstances or usages of “at least one” or “one or more.” In thisdocument, the term “or” is used to refer to a nonexclusive or, such that“A or B” includes “A but not B,” “B but not A,” and “A and B,” unlessotherwise indicated. In this document, the terms “including” and “inwhich” are used as the plain-English equivalents of the respective terms“comprising” and “wherein.” Also, in the following claims, the terms“including” and “comprising” are open-ended, that is, a system, device,article, composition, formulation, or process that includes elements inaddition to those listed after such a term in a claim are still deemedto fall within the scope of that claim. Moreover, in the followingclaims, the terms “first,” “second,” and “third,” etc. are used merelyas labels, and are not intended to impose numerical requirements ontheir objects.

Method examples described herein can be machine or computer-implementedat least in part. Some examples can include a computer-readable mediumor machine-readable medium encoded with instructions operable toconfigure an electronic device to perform methods as described in theabove examples. An implementation of such methods can include code, suchas microcode, assembly language code, a higher-level language code, orthe like. Such code can include computer readable instructions forperforming various methods. The code may form portions of computerprogram products. Further, in an example, the code can be tangiblystored on one or more volatile, non-transitory, or non-volatile tangiblecomputer-readable media, such as during execution or at other times.Examples of these tangible computer-readable media can include, but arenot limited to, hard disks, removable magnetic disks, removable opticaldisks (e.g., compact disks and digital video disks), magnetic cassettes,memory cards or sticks, random access memories (RAMs), read onlymemories (ROMs), and the like.

The above description is intended to be illustrative, and notrestrictive. For example, the above-described examples (or one or moreaspects thereof) may be used in combination with each other. Otherembodiments can be used, such as by one of ordinary skill in the artupon reviewing the above description. The Abstract is provided to complywith 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain thenature of the technical disclosure. It is submitted with theunderstanding that it will not be used to interpret or limit the scopeor meaning of the claims. Also, in the above Detailed Description,various features may be grouped together to streamline the disclosure.This should not be interpreted as intending that an unclaimed disclosedfeature is essential to any claim. Rather, inventive subject matter maylie in less than all features of a particular disclosed embodiment.Thus, the following claims are hereby incorporated into the DetailedDescription as examples or embodiments, with each claim standing on itsown as a separate embodiment, and it is contemplated that suchembodiments can be combined with each other in various combinations orpermutations. The scope of the invention should be determined withreference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

What is claimed is:
 1. In a network having a first and second node,wherein each node includes beamforming coefficients, a method ofdetermining the location of a node, the method comprising: determiningthe beamforming coefficients associated with signals transmitted fromthe first node to the second node; determining a direction of the secondnode using the beamforming coefficients; determining a distance betweenthe first node and the second node as a function of time of flight anddirection; and determining a location of the second node as a functionof distance and direction.
 2. The method of claim 1, wherein determininga distance includes determining a round trip delay of a signal from oneof the first and second nodes to the other of the first and secondnodes.
 3. The method of claim 1, wherein determining a distance includesmaking a four-way time-of-arrival (TOA) determination
 4. The method ofclaim 1, wherein determining a distance includes determining time offlight as a function of a continuous distribution of time of flightmeasurements.
 5. The method of claim 1, wherein determining a distanceincludes monitoring transmissions between the first and second nodes viaa third node; and calculating, based on measurements at the second node,a time of flight between the first and second nodes.
 6. The method ofclaim 1, wherein determining a distance includes determining if thefirst or second node is moving and adjusting direction accordingly. 7.The method of claim 1, wherein determining direction includesdetermining direction using each of two different bands.
 8. In a networkhaving including a first, second and third node, wherein each nodeincludes beamforming coefficients, a method of determining the locationof a node, the method comprising: determining the beamformingcoefficients associated with signals transmitted from the first node tothe second node and the beamforming coefficients associated with signalstransmitted from the third node to the second node; determining adirection of the second node from the first and third nodes using thebeamforming coefficients; and determining a location of the second nodeas a function of direction from each of the first and third nodes andthe location of the first and third nodes.
 9. The method of claim 8,wherein determining a location includes: determining a distance betweenthe first node and the second node as a function of time of flight anddirection; determining a distance between the third node and the secondnode as a function of time of flight and direction; and determining alocation of the second node as a function of direction and distance fromeach of the first and third nodes and of triangulation from the firstand third nodes.
 10. The method of claim 9, wherein determining adistance includes determining round trip delay of a signal from one ofthe first and second nodes to the other of the first and second nodes.11. The method of claim 9, wherein determining a distance includesmaking a four-way time-of-arrival (TOA) determination.
 12. The method ofclaim 9, wherein determining a distance includes determining a time offlight as a function of a continuous distribution of time of flightmeasurements.
 13. The method of claim 9, wherein determining a distanceincludes monitoring transmissions between the first and second nodes viaa third node; and calculating, based on measurements at the second node,time of flight between the first and second nodes.
 14. The method ofclaim 9, wherein determining a distance includes determining if thefirst or second node is moving and adjusting direction accordingly. 15.The method of claim 8, wherein determining a direction includesdetermining a direction using each of two different bands.